A generally used characteristic parameter for indicating the quality of diesel fuel is the cetane number, which is, for a given engine compression rate, a measure of the time delay between injection and ignition of the fuel in the engine. A standard test engine, also referred to as CFR engine, is applied for such measurements. Apart from the time delay, the cylinder pressure is measured during a selected time interval of the compression stroke of the engine. The part of the engine cycle of interest generally is between 13.degree. before top dead center (BTDC) and top dead center (TDC), and it is convenient to start data sampling at 30.degree. BTDC compression.
In order to generate a trigger signal indicating 30.degree. BTDC compression, it has already been proposed to apply an optical shaft encoder which generates crank angle degree markers and a reference pulse which is usually aligned, when fitted to the engine, with TDC. To only generate one pulse at 30.degree. BTDC of the compression stroke of each engine cycle (two revolutions of the encoder shaft) the encoder is provided with internal gearing. The encoder then has to be carefully aligned with the engine so that the once per two revolutions signal occurs at 30.degree. BTDC compression. This once per two revolutions signal could of course be set to occur anywhere in the engine cycle and it is only through careful alignment that it occurs at the required 30.degree. BTDC compression. A drawback of such system is the requirement for a non-standard encoder. Moreover exact alignment of the encoder with the engine is difficult to achieve or to maintain.